The invention relates to a multi-wire electric power cable, particularly a supply cable for borehole units, for example pumps, in which each of the wires which lie alongside of the other is protected individually by a closed metallic covering in the form of a corrugated tube.
For the production of oil or gas wells drive elements are used which are introduced to depths of 3000 m and more. These drive elements, particularly pumps, are provided with electric power from the surface of the earth. For this purpose, electric cables are required which must satisfy very specific requirements. There must be taken into account, on the one hand, the pressure conditions which prevail at these depths and which require that the cable be correspondingly resistant to pressure, while, furthermore, account must also be taken of temperatures which are about 120.degree. C. and more, in addition to the temperatures produced by the drive elements in the form of heat loss. Another requirement for the operation of the cables over a long period of time is a substantial lack of sensitivity to corrosive agents present in the borehole or shaft, such as, for instance, corrosive gases or else seawater when the holes are made in the bottom of the sea.
From Federal Republic of Germany OS No. 2853100 a cable for this purpose is known in which each wire is surrounded by a corrugated metal sheath. The individual adjacent wires surrounded by a corrugated sheath are, in accordance with a further concept of the known arrangement, themselves surrounded by a corrugated metal sheath. Since this corrugated sheath is necessarily round as a result of the process, there results for the cable a diameter which permits the use of this cable in only a few cases. For example, if the space between the borehole casing and the so-called drill pipe to which the cable is customarily fastened is very small, the known construction cannot be used. Furthermore, the relatively large corrugated sheathing leads to poorer resistance of the structure to pressure. In accordance with another proposal made in the foregoing Federal Republic of Germany OS No. 2853100, each corrugated tube surrounding the adjacent wires is surrounded by an armoring of high-strength steel wires, over which a tape wrapping which is common to all wires is then applied. The armoring of high-strength steel wires serves essentially to take up tensile forces. The purpose of the tape wrapping is to keep the three wires in the desired position. In order to achieve resistance to pressures on the order of magnitude of 200 bar with this construction, it is necessary, in the case of a predetermined inside diameter of the corrugated tube, which is determined by the outside diameter of the electrical insulation of the wires, to increase the wall thickness of the corrugated tube, as a result of which, however, the flexibility of the cable is substantially reduced. The suggestion of using a so-called double corrugated sheathing also failed to lead to a substantially higher pressure-withstanding capacity of the corrugated tube.
It is an object of the invention to improve the cable of the aforementioned type so as to obtain a resistance to pressures of more than 200 bar, and preferably more than 300 bar, without great expense. In addition, the cable is to have a sufficiently high longitudinal tensile strength.